Reset Factory Radio

I have an 04 SLT Quad Cab, I removed my aftermarket radio getting ready to sell the truck. But when I reinstalled the factory radio it does not work. The only thing I can get to work are the dimmer lights.

I probably need the security code, but have no time to go to the dealer to get it.

If you have a late model '04, the computer bus may be integrated with the audio system. If that was bypassed to install the aftermarket radio, you may have to go to the dealer to get it resolved. Are the factory speakers still in the truck? Or, aftermarket? Could be if they are aftermarket, the factory radio does not have enough oomph to drive the speakers.